Studies are made to identify persons at very high risk of cancer and to investigate causes of their susceptibility. The individuals are found through special referrals by clinicians, self-referral by patients, and clinical observations at patient conferences, rounds and clinics. With informed consent, epidemiologic studies are made to identify the predisposing role of cancer genes and environmental carcinogens. Laboratory studies asre performed to clarify biologic mechanisms of susceptibility to cancer. Findings to date show that cancer risk approaches l00% in persons who are carriers of certain cancer genes. Early cancer detection has been achieved in some patients, and appropriate counseling during pre-clinical stages has been provided to other patients. High risk groups include patients who had one cancer in childhood and are susceptible to develope multiple promary neoplasm. Cancesr risk factors in these patients are under prospective study through a computerized Registry of Survivors of Childhood Cancer in Boston.